The present invention relates in general to crankcase ventilation for internal combustion engines, and, more specifically, to ventilation of a gasoline engine that employs a turbocharger for compressing the intake air at high engine loads.
Gases accumulate in an engine crankcase when gases from engine cylinders bypass engine pistons and enter the crankcase during engine rotation. These gases are commonly referred to as blowby gases. The blowby gases can be combusted within engine cylinders to reduce engine hydrocarbon emissions using a positive crankcase ventilation (PCV) system which returns the blowby gases to the engine air intake and combusting the gases with a fresh air-fuel mixture. Combusting crankcase gases via the engine cylinders may require a motive force to move the crankcase gases from the engine crankcase to the engine air intake. One conventional way to provide motive force to move crankcase gases into the engine cylinders is to provide a conduit between the crankcase and a low pressure region (e.g., vacuum) of the engine intake manifold downstream of an engine throttle body. In addition, fresh air from a point upstream of the throttle body is added to the crankcase via a separate conduit (i.e., breather) to help flush the blowby products from the crankcase and into the intake manifold.
Use of turbocharging with combustion engines is becoming increasingly prevalent. In an exhaust-gas turbocharger, for example, a compressor and a turbine are arranged on the same shaft (called a charger shaft) wherein a hot exhaust-gas flow supplied to the turbine expands within the turbine to release energy and cause the charger shaft to rotate. The charger shaft drives a compressor which is likewise arranged on the charger shaft. The compressor is connected in an air inlet duct between an air induction and filtering system and the engine intake manifold so that when the turbocharger is activated, the charge air supplied to the intake manifold and engine cylinders is compressed.
Turbocharging increases the power of the internal combustion engine because a greater air mass is supplied to each cylinder. The fuel mass and the mean effective pressure are increased, thus improving volumetric power output. Accordingly, the engine displacement used for any particular vehicle can be downsized in order to operate with increased efficiency and reduced fuel use, wherein the turbocharger is inactive during times of low power requirements and is activated during times of high load, such as wide open throttle (WOT). In addition to reduced fuel consumption, turbocharging has a beneficial effect of reducing emissions of carbon dioxide and pollutants.
Due to the increased pressure at the intake manifold during high load operation which results from compressing the inlet air by the turbocharger compressor, modifications to the conventional crankcase ventilation system are necessary. In particular, the high pressure introduced downstream of the compressor (e.g., in the intake manifold) could reverse the flow in the vent line thereby pressurizing the crankcase to an extent that could cause failure of the seals. To prevent such a reversal, a check valve is usually placed in that vent line. To avoid a buildup of blowby gas in the crankcase, the flow is allowed to reverse in the other vent line (i.e., the breather that otherwise supplies fresh air from a point upstream of the throttle body and turbocharger compressor into the crankcase). Thus, any pressure buildup in the crankcase that could damage the seals is prevented.
During engine idling when a large vacuum is present at the intake manifold, it is desirable to maintain a negative pressure in the crankcase. To ensure a negative crankcase pressure at idle on a boosted gas (i.e., turbocharged) engine, it is often necessary to restrict the fresh air feed to the crankcase. An appropriately sized restriction in the corresponding breather vent line is used to accomplish this. However, if the crankcase fresh air feed is restricted too much then the crankcase may become positively pressurized under full load conditions (i.e., when the restricted vent line or breather reverses flow to evacuate the blowby gases into the low pressure section of the air inlet system), which can jeopardize the crankcase sealing integrity. It is often difficult or impossible to find a restriction level that provides the needed vacuum at idle while not creating an undesirably large positive pressure during full load operation.
Copending U.S. application Ser. No. 14/525,554, filed Oct. 28, 2014, entitled “Crankcase Ventilation for Turbocharged Engine,” incorporated herein by reference, discloses a dual-acting valve having a first flow capacity into the crankcase and a second flow capacity out from the crankcase which is greater than the first flow capacity. The dual-acting valve provides the desired restriction when the engine is in an idle state and provides a greater flow when the engine is in a boosted state (i.e., when the turbocharger pressurizes the intake manifold) to avoid over-pressurization of the crankcase. In such a system, however, undiluted blowby gases are collected to be ingested by the engine. Oil degradation such as sludging, varnishing, and emulsification can occur due to insufficient fresh air being mixed with the blowby gases in the crankcase prior to reaching the oil separator. Undiluted blowby gases may accumulate high levels of unburned fuel, such as during a decel fuel cutoff, which may increase pollution or cause other problems.